1. Field of the Invention
The present invention relates generally to image reading apparatus such as image scanners, and more particularly to an image reading apparatus of the so-called "flat bed" type which is capable of reading a stationary original sheet and a moving original sheet by scanning the original sheet with a movable optical unit including a light source and a mirror and by reading scanned information on the original sheet as two-dimensional information by a fixed photoelectric conversion unit.
2. Description of the Prior Art
A conventional image reading apparatus of the type concerned includes, as shown in FIGS. 9 and 10, a housing or case 1, an original holding plate 2 made of transparent glass, a chassis 3 attached horizontally to the case 1, a pair of parallel spaced guide members 4 attached to the chassis 3 and extending in a plane parallel to the original holding plate 2, a first movable optical reading unit 5 movably mounted on the guide members 4 and having a light source (not shown) and a mirror 6, a second movable optical reading unit 7 movably mounted on the guide members 4 and having two mirrors 8 and 9, a photoelectric conversion unit 10 fixedly mounted on the chassis 3 and having a lens 11 and an image sensor 12, and a drive mechanism (not shown) for moving the first and second optical reading units 5 and 7 along the guide members 4. In general, the chassis 3 is molded of a synthetic resin such as acrylonitrile-butadiene-styrene (ABS) resin, and the guide members 4 are elongate metal plates.
The image reading apparatus is so constructed as to deal with both a stationary original sheet which is manually placed on the original holding plate 2, and a moving original sheet which is automatically supplied onto the original holding plate 2 by an automatic document feeder described later on. To this end, as shown in FIG. 11, the original holding plate 2 is composed of a first flat bed glass 2a for holding thereon a stationary original sheet 13 (FIG. 9) to be read, and a second flat bed glass 2b for guiding therealong a moving original sheet 13 as it passes through a moving original reading position located on the second flat bed glass 2b. The first movable optical reading unit 5 incorporating therein the light source 14 and the mirror 6 is locatable directly below the moving original reading position. The first and second flat bed glasses 2a and 2b are attached to the case 1 by a pair of bed glass support members 15 and 16 so that the first and second flat bed glasses 2a and 2b are flush with each other or extend in the same plane.
An automatic document feeder 17 which is an optional part of the image reading apparatus is disposed above the original holding plate 2 for automatically feeding original sheets 13 one after another through the moving document reading position. The automatic document feeder 17 includes an original conveyor roller system 18 composed of a plurality of mating roller pairs for feeding the original sheet 13 onto said second flat bed glass 2b and subsequently discharging the original sheet 13 downstream from said second flat bed glass 2b, a pair of confronting entrance guide members 19 and 20 for guiding the original sheet 13 over and across the second flat bed glass 2b while the original sheet 13 is being conveyed by the original conveyor roller system 18, and an exit guide 21 for guiding the original sheet 13 as the original sheet 13 is discharged from the moving document reading position on the second flat bed glass 2b to the outside of the automatic document feeder 17.
When the image reading apparatus is used with a stationary original sheet as a stationary original exposure type image reading apparatus, an original sheet 13 is placed on the first flat bed glass 2a (FIG. 11) of the original holding plate 2, as shown in FIG. 9. Then, the original sheet 13 is irradiated with the light source 14 (FIG. 11), and light reflected from the original sheet 13 is guided by the mirrors 6, 8 and 9 into the photoelectric conversion unit 10. The first optical reading unit 5 is moved along the entire surface of the original sheet 13 and, in synchronism with the movement with the first optical reading unit 5, the second optical reading unit 7 is moved so that the entire surface of the original sheet is scanned and thus information is read from the original sheet.
When the components of the image reading apparatus are thermally deformed due to a change in environmental conditions (temperature change, for example), the amount of thermal deformation of the chassis 3 is greater than that of the guide members 4 because, as described above, the chassis 3 is molded of a synthetic resin and the guide members 4 are made of metal. Due to the difference in thermal deformation, the chassis 3 tends to deform or bend, thus displacing the photoelectric converting unit 10 held thereon relative to the guide members 4. As a result, an optical axis which is defined by the mirrors 6, 8 and 9 of the first and second optical reading units 5 and 7 held on the guide members 4 is displaced out of alignment with an optical axis of the photoelectric conversion unit 10, so that the quality of an image read on the image reading apparatus is deteriorated.
Furthermore, when the chassis 3 is twisted or otherwise deformed by an external force or pressure, the photoelectric conversion unit 10 is displaced relative to the guide members 4 on which the first and second optical reading units 5 and 7 are held. Thus, the above-mentioned misaligned optical axes problem occurs again.
When the image reading apparatus is used with a moving original sheet as a moving original exposure type image reading apparatus, a leading one 13 of original sheets set on the automatic document feeder 17 is supplied by the original conveyor roller system 18 and then guided by the entrance guide members 19 and 20 so that the original sheet 13 passes through the moving original reading position on the second flat bed glass 2b in a sub-scanning direction, as shown in FIG. 11. During that time, the light source 14 of the first optical reading unit 5 held at a position directly below the second flat bed glass 2b irradiates the moving original sheet 13. Light reflected from the moving original sheet 13 is directed by the mirror 6 in a direction parallel to the original holding plate 2 as indicated by the arrow 22 and then it is further directed by the mirrors 8 and 9 (FIG. 9) toward the photoelectric conversion unit 10 (FIG. 10) so that information is read from the original sheet 13. The original sheet 13 is subsequently guided by the exit guide 21 and discharged to the outside of the automatic document feeder 17.
In the moving original reading operation described above, it is important that the original sheet 13 after having been read above the second flat bed glass 2b is smoothly and stably guided by the exit guide 21 and then discharged to the outside of the automatic document feeder 17. To this end, the first flat bed glass 2a and the second flat bed glass 2b are separated by a space so that this space can be used for setting a front end (upstream end) of the exit guide 21 at a level below an upper surface of the second flat bed glass 2b. Instead of using two flat bed glasses 2a and 2b, a single flat bed glass 2c may be used as shown in FIG. 12 in which instance, however, the upper surface of the flat bed glass 2c must be recessed at a portion immediately downstream of the moving original reading position. A guide member 23 is disposed in the recess for guiding the original sheet 13 toward the outside of the automatic document feeder 17 after the original sheet 13 moves past the moving document reading position on the flat bed glass 2c.
The image reading apparatus having two flat bed glasses 2a and 2b such as shown in FIG. 11 is costly to manufacture due to the use of two flat bed glasses 2a and 2b. In addition, in order to attach the two flat bed glasses 2a and 2b, the image reading apparatus requires additional bed glass support members 15, 16 which makes the case complicated in construction and increase the overall size of the image reading apparatus. The image reading apparatus having a single flat bed glass 2c such as shown in FIG. 12 is free from the foregoing problems caused by the use of two flat bed glasses 2a and 2b, however, due to the necessity of providing the recess, the flat bed glass 2c is costly to manufacture and eventually increases the overall production cost of the image reading apparatus.